The distribution of nitrogen in early Solar System materials is crucial for understanding its chemical evolution. Remote sensing suggests that comets and icy bodies in the outer Solar System possess significant inventories of solid nitrogen compounds, such as ammonium salts. However, the connection between this outer Solar System nitrogen and the near-Earth region remains unclear. Ammonium salts, while potentially stable in the near-Earth environment, haven't been detected in interplanetary dust particles or Antarctic micrometeorites, which are thought to originate from these outer Solar System bodies. Isotopic analysis of lunar ilmenite indicates that exogenous nitrogen on the Moon comes from asteroidal micrometeoroids, but the nitrogen abundance in these meteoroids remains unquantified. The Hayabusa2 mission to the near-Earth C-type asteroid Ryugu provided a unique opportunity to investigate this question. Ryugu's samples, being chemically primitive and similar to CI carbonaceous chondrites, offer insights into the early Solar System and the potential transport of nitrogen from the outer regions. Space weathering processes, including micrometeoroid impacts and solar wind implantation, have modified the Ryugu samples, and studying these modifications can provide clues about the supply and properties of primitive material to Ryugu's orbit. While metallic iron is a common product of space weathering on airless bodies, its interaction with exogenous materials, especially nitrogen, has not been extensively investigated. This study focuses on analyzing iron sulfides, magnetite, and carbonate—the major iron-bearing minerals in Ryugu samples—to understand the chemical state of iron due to space weathering and its potential role in nitrogen fixation.

Previous research highlighted the presence of significant nitrogen compounds in comets and icy bodies in the outer Solar System. Studies using remote sensing techniques suggested that ammonium salts are a major reservoir of nitrogen in these objects. However, a direct link between these outer Solar System nitrogen reservoirs and the near-Earth region has been missing. Analyses of interplanetary dust particles and Antarctic micrometeorites, potential carriers of outer Solar System material, have failed to identify significant amounts of ammonium compounds. Studies on the Moon have shown an exogenous nitrogen supply from infalling asteroidal micrometeoroids, but precise quantification of nitrogen abundance in these meteoroids was lacking. The chemical composition of Ryugu samples, consistent with CI carbonaceous chondrites, and the space weathering effects they experienced, were identified as potential keys to unlocking this issue. Previous investigations of lunar and Itokawa samples showed that metallic iron is a common product of space weathering and its formation through the selective escape of volatile elements was proposed. However, how this metallic iron interacts with exogenous components, including nitrogen, on the surface of airless bodies was unclear and remains an open question.

The study examined space-exposed surfaces of Ryugu grains, including magnetite, iron sulfides, and carbonate, using various techniques including scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and transmission electron microscopy (TEM). SEM was used to identify Ryugu grains showing surface modifications related to space weathering, characterized by granular surfaces on magnetite and depressions/pores on iron sulfides. STEM was employed for detailed analysis of the surface modifications of magnetite grains, focusing on elemental mapping and determining the Fe/O ratio. The presence of iron-rich layers enriched in silicon, magnesium, sulfur, and nitrogen was investigated using STEM-EDX. Nanobeam electron diffraction (NBD) patterns were obtained to identify the crystalline phases present in these layers, with a particular focus on the detection of iron nitrides. TEM was utilized to study the crystallographic structure and chemical composition of the modified layers, providing insights into the formation mechanisms of metallic iron and its nitridation. Electron energy-loss spectroscopy (EELS) was used to determine the oxidation state of iron in the iron-rich layer. The study also investigated the surface modifications of iron sulfides and carbonate, analyzing their microstructure, elemental composition, and possible volatile escape processes. The coexistence of modified magnetite and carbonate with space-weathered sulfides and phyllosilicates, along with the presence of impact craters and melted deposits, was examined as further evidence of space exposure. The depth of crystallographic damage caused by solar wind irradiation was evaluated and compared to the penetration depth of solar wind particles. The experimental methods also included a calculation to estimate the nitrogen accumulation from the solar wind and an evaluation of the nitridation of iron metal by ammonia gas in impact vapor produced under various conditions. Helium ion irradiation experiments were performed on Ryugu grains to simulate solar wind effects and understand the modifications in magnetite.

The key finding of the study is the discovery of iron nitride (Fe4N) on the surface of magnetite grains from asteroid Ryugu. This iron nitride is present in granular iron-rich layers that cover the magnetite surface. These layers are enriched not only in iron and oxygen but also in significant amounts of silicon, magnesium, sulfur, and nitrogen. The presence of a cubic crystal with lattice parameters consistent with roaldite (Fe,N) was confirmed using NBD patterns. The Fe/O ratio in the magnetite was found to increase towards the uppermost iron-rich layers, suggesting the selective loss of oxygen due to processes like ion sputtering and thermal effects of micrometeoroid impacts. The analysis also showed that metallic iron is present on both magnetite and iron sulfides, indicating that the decomposition of iron-bearing minerals by space weathering is independent of the overall oxidation state of the asteroid. The analysis of iron sulfides revealed the presence of iron whiskers, composed of bcc and fcc iron, suggesting sulfur escape through solar wind implantation, micrometeoroid bombardment, or thermal cycling. The conversion of non-stoichiometric pyrrhotites to troilite is also consistent with sulfur loss. The modified carbonate grains showed a ferropericlase rim and selective escape of carbon and oxygen without complete reduction to metallic iron. The presence of nitrogen in the iron-rich layers was a key finding, contrasting with the depletion of other volatile elements (H, C, O, S). The study estimated the supply rate of nitrogen atoms to the magnetite surface to be 4–6 × 10⁵ atoms cm⁻²s⁻¹, significantly higher than the rate estimated for lunar ilmenite. This implies that the source of nitrogen is not solely the solar wind, and the high concentration of nitrogen suggests an exogenous source or sources. The absence of evidence supporting solar wind or IOM sputtering as the major cause of nitridation points toward micrometeoroid impacts as a likely cause. The presence of sulfur in the iron-rich layers also supports the possibility of the exogenous sources having NHx salts.

The discovery of iron nitride on Ryugu's magnetite grains directly addresses the research question about the transport of nitrogen into the inner Solar System. The presence of iron nitride strongly suggests that nitrogen-rich materials from the outer Solar System contributed to the composition of Ryugu. The high nitrogen supply rate and the absence of evidence for solar wind implantation as the primary nitrogen source strongly suggest an influx of exogenous material rich in nitrogen. This could be due to either dust impacts or larger micrometeoroid impacts from nitrogen-rich outer Solar System sources. The findings challenge the previous understanding of nitrogen availability in the inner Solar System, implying that the abundance of nitrogen during planetary formation and the early stages of life's emergence may have been higher than previously recognized. The presence of significant amounts of nitrogen in refractory solids delivered to Ryugu by impactors significantly impacts our understanding of the volatile delivery process in the early Solar System and could have implications for the origin and early evolution of life on Earth. Further research using isotopic analysis of nitrogen in Ryugu samples could refine our understanding of the source regions and delivery mechanisms of this exogenous nitrogen.

This study provides compelling evidence for the influx of nitrogen-rich material from the outer Solar System, based on the discovery of iron nitride on magnetite grains from asteroid Ryugu. The high concentration of nitrogen in iron-rich layers and the estimated nitrogen supply rates strongly support an exogenous origin. This suggests that the amount of nitrogen available for planetary formation and prebiotic reactions in the inner Solar System is likely greater than previously recognized. Future investigations focusing on isotopic analysis of nitrogen and further characterization of the exogenous material will provide crucial insights into the origin and delivery of this nitrogen. The research highlights the importance of studying returned asteroid samples to understand the early Solar System's volatile inventory.

The study focused on a limited number of Ryugu samples, and a larger sample size could provide more statistically robust results. The exact composition of the impactors and their specific nitrogen-bearing compounds remain unclear. While the findings suggest an exogenous origin for the nitrogen, it is still important to examine the role of Ryugu's internal materials in the nitridation process. The nitrogen isotopic ratios could provide further insights into the origin and delivery of this nitrogen; however, they were not included in this study.